The present invention relates to an optical sampling waveform measuring apparatus and, more particularly, to an optical sampling waveform measuring apparatus for measuring the pulse waveform of an optical signal used for optical communication or the like by using sum frequency light or sum frequency generation, and a polarization beam splitter which can be assembled in the measuring apparatus.
In constructing a new optical communication system, manufacturing a new optical transmission system, or periodically inspecting such an optical communication system or optical transmission system to check the quality of optical communication, it is important to measure the pulse waveform of a digital optical signal to be transmitted/received.
As a conventional technique of measuring the pulse waveform of such an optical signal, a technique of converting an optical signal including a pulse waveform into an electrical signal by using, e.g., a photoelectric converter, and displaying the pulse waveform converted into the electrical signal on the display screen of an oscilloscope is known.
Recently, with an increase in the transmission rate of information by means of optical signals, a high-speed optical transmission scheme with a transmission rate of 10 Gbit/s or more has been planned.
It is difficult to measure a high-speed optical signal exceeding several 10 Gbit/s with the response speed of an existing photoelectric converter for converting an optical signal into an electrical signal.
To solve this problem, an optical sampling waveform measuring apparatus for measuring the pulse waveform of an optical signal by using sum frequency light has been developed (Jpn. Pat. Appln. KOKOKU Publication No. 6-63869 which is published in Jpn. Pat. Appln. KOKAI Publication No. 2-311732).
FIGS. 15A, 15B, 15C, 16A, and 16B are views for explaining the principle of measurement by this optical sampling waveform measuring apparatus.
Assume that target light (to be measured) a having a pulse waveform to be measured and a repetition frequency f and sampling light b having a pulse width much smaller than that of the target light a and a repetition frequency (f-.DELTA.f) slightly lower (or higher) than the repetition frequency f of the target light a are incident on a nonlinear optical member 1 for type II phase matching (which is called type II, and hence will be referred to as type II hereinafter).
Sum frequency light c proportional to the product of the intensities of the light a and the light b is output from the nonlinear optical member 1 only when the light a and the light b incident on the nonlinear optical member 1 are simultaneously superimposed on the same optical axis of the nonlinear optical member 1.
The repetition frequency of this sum frequency light c is equal to the repetition frequency (f-.DELTA.f) of the sampling light b.
It therefore suffices if the response speed of the photoelectric converter used in this optical sampling waveform measuring apparatus is higher than this repetition frequency (f-.DELTA.f). In addition, since the time resolution is determined by the pulse width of the sampling light b, according to this optical sampling waveform measuring apparatus, the pulse waveform of the target light a which has broadened in the time axis direction can be accurately measured, as shown in FIGS. 15A, 15B, and 15C.
That is, as shown in FIG. 16A, when the target light a having an angular frequency .omega..sub.D and the sampling light b having an angular frequency .omega..sub.S are incident on one surface of the type II nonlinear optical member 1 in directions in which the planes of polarization of the light a and the light b are orthogonal to each other, sum frequency light c having a sum angular frequency (.omega..sub.S +.omega..sub.D) is output from the other surface of the member 1 under the condition in which the light a and the light b are phase-matched.
FIG. 16B is a graph showing the relationship between angular frequencies .omega. and optical powers P of the target light a, the sampling light b, and the sum frequency light c.
Letting P.sub.SFG be the intensity of the sum frequency light c and P.sub.SIG and P.sub.SAM be the intensities of the target light a and the sampling light b, the intensity P.sub.SFG of the sum frequency light c is generally expressed by EQU P.sub.SFG =.eta..multidot.P.sub.SIG .multidot.P.sub.SAM (1)
where .eta. is the nonlinear conversion efficiency constant that is uniquely determined by the type or material of the nonlinear optical member 1 used.
This nonlinear conversion efficiency constant .eta. is generally as small as, for example, 10.sup.-5 to 10.sup.-4.
The optical sampling waveform measuring apparatus must therefore use a nonlinear optical member 1 whose nonlinear conversion efficiency constant .eta. is large.
In the optical sampling waveform measuring apparatus, KDP (KH.sub.2 PO.sub.4), LN (LiNbO.sub.3), LT (LiTaO.sub.3), KN (KNbO.sub.3), or the like is used as the nonlinear optical member 1.
FIG. 17 is a block diagram showing the schematic arrangement of the optical sampling waveform measuring apparatus incorporating the type II nonlinear optical member 1 described above.
The externally input continuous target light a having the angular frequency .omega..sub.D and the pulse waveform repeating frequency f is controlled by a polarization direction controller 2 to have a plane of polarization in a 90.degree. direction with respect to a reference direction (0.degree. direction). The resultant light enters a multiplexer 3.
Meanwhile, a sampling light source 4 outputs the continuous sampling light b having the angular frequency .omega..sub.S different from the angular frequency .omega..sub.D of the target light a and the pulse waveform repeating frequency (f-.DELTA.f).
As shown in FIG. 15B, the pulse width of the sampling light b is set to be much smaller than that of the target light a.
The sampling light b output from the sampling light source 4 is controlled by a polarization direction controller 5 to have a plane of polarization, for example, in the reference direction (0.degree. direction). The resultant light also enters the multiplexer 3.
The multiplexer 3 composed of, for example, a beam splitter (BS) and the like allows incident light to propagate straight and reflects incident light in a right-angle direction by using a half mirror 3a.
With this arrangement, the sampling light b having a plane of polarization in the reference direction (0.degree. direction) and the target light a having a plane of polarization in a 90.degree. direction with respect to the reference direction (0.degree. direction) are simultaneously incident on one surface of the type II nonlinear optical member 1 that is placed on the optical axis of the sampling light b behind the multiplexer 3.
The sum frequency light c having the angular frequency (.omega..sub.S +.omega..sub.D) is therefore output from the other surface of the type II nonlinear optical member 1.
The sum frequency light c output from the nonlinear optical member 1 is incident on a photodetector 7 through an optical filter 6.
Note that the sum frequency light c output from the nonlinear optical member 1 includes light components having angular frequencies 2.omega..sub.S and 2.omega..sub.D which are twice the angular frequencies .omega..sub.S and .omega..sub.D, respectively, and unconverted light components having the angular frequencies .omega..sub.S and .omega..sub.D, albeit slightly, in addition to a light component having the sum angular frequency (.omega..sub.S +.omega..sub.D) of the angular frequencies .omega..sub.S and .omega..sub.D of the light b and the light a.
The optical filter 6 removes these light components having the angular frequencies 2.omega..sub.S, 2.omega..sub.D, .omega..sub.S, and .omega..sub.D.
The photodetector 7 is a photoelectric converter or the like and serves to convert the sum frequency light c into an electrical signal d and output it to an electrical signal processing system 8 on the next stage.
The electrical signal processing system 8 generates the optical pulse waveform of the target light a, which has broadened in the time axis direction, from the input electrical signal d having the same waveform as that of the sum frequency light c like the one shown in FIG. 15C by the method described above. The electrical signal processing system 8 then displays the resultant waveform on a display 9.
In the optical sampling waveform measuring apparatus shown in FIG. 17, however, the following problems are left unsolved.
The intensity P.sub.SFG of the sum frequency light c output from the type II nonlinear optical member 1 is expressed as the product of the intensity P.sub.SAM of the reference-direction (e.g., 0.degree.-direction) light component of the sampling light input to the nonlinear optical member 1, the intensity P.sub.SIG of the light component in the 90.degree. direction with respect to the reference direction of the target light a, and the nonlinear conversion efficiency .eta., as has been described with reference to equation (1).
That is, in this optical sampling waveform measuring apparatus shown in FIG. 17, when the plane of polarization of the sampling light b input to the nonlinear optical member 1 and the plane of polarization of the target light a are linearly polarized, and their planes of polarization are perfectly orthogonal to each other, the intensity P.sub.SFG of the sum frequency light c is maximized.
When, therefore, the optical pulse waveform of target light is to be stably measured with maximum sensitivity, the respective polarized states must be adjusted by the polarization direction controllers 2 and 5 to stabilize before measurement of the optical pulse waveform of the target light a so as to maximize the intensity P.sub.SFG of the sum frequency light c (satisfy the above condition).
In addition, since the distance between the sampling light source 4 and the polarization direction controller 5 is short, the polarized state of the sampling light b impinging on the polarization direction controller 5 varies slowly, and hence can be maintained constant within almost the measurement time.
If, however, the target light a is a signal sent through a long-distance single-mode optical fiber for a communication network or a signal sent through the interconnection in an optical communication device in an unstable temperature state, the polarized state of the light greatly varies with time and deviates from the state adjusted before measurement. This may cause variations in measured waveform within a short period of time.
After the start of measurement, therefore, when the polarized state of the target light a changes, the 90.degree.-direction light component of the target light a output from the polarization direction controller 2 becomes small in amount unless the polarization direction controllers 2 and 5 are adjusted occasionally.
This means that the intensity P.sub.SIG of the target light a reflected by the half mirror 3a of the multiplexer 3 and input to the nonlinear optical member 1 varies. This leads to a deterioration in measurement precision for the optical pulse waveform of target light.
Although the polarized state of each light component can be adjusted occasionally after the start of measurement, it is impossible in practice to instantaneously determine whether the signal intensity has decreased or the polarized state has deviated. Furthermore, adjustment must be performed frequently during the measurement, making the apparatus impractical.